Why is the pre-existing conditions provision a part of health bill anyway?

I must be missing something about the possible health care bill, which may or not soon be brought up for a vote in Congress. I don’t understand why the pre-existing conditions provision should even be part of the bill. This would be akin to driving your car without insurance until you destroy or hurt someone with it and then call an insurance agent the next day to buy insurance.

I would be glad to hear a good argument for having this provision in the bill, but it seems to me that this changes the definition of “insurance.”